International Conference Ozone and UV, April 3rd 2006
Oil Shale Semicoke Leachate Pre-treatment by means of
Advanced Oxidation
Niina Kulik, Marina Trapido, Yelena Veressinina, Rein Munter
Department of Chemical Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn,
Estonia; e-mail: niina.kulik@mail.ee
Keywords: Oil shale, Leachate, Fenton treatment, Ozone
Introduction
Oil shale is one of the most important natural resources in Estonia, and it is widely used in the
chemical industry and in energy production. More than 80 years of oil shale thermal processing
has resulted in huge dump sites of the coke ash (semicoke) from semicoking of oil shale in the
areas surrounding oil shale chemical industry plants in north-eastern part of Estonia. The
semicoke dumps cover an area about 200 ha and contain up to 100 million tons of solid waste.
Currently, about 600000 tons of processed semicoke is disposed annually [1,2].
Leachate is formed when the semicoke from the processing of oil shale is deposited in dumps
under the influence of precipitation, drainage and process water pumped onto the hills in order
to compact them. Semicoke solid wastes contain several organic and inorganic compounds (oil
products, asphaltenes, phenols, PAHs, sulphuric compounds). The leachate from these dumps,
formed from flushing water and precipitation, contains variety of organic and inorganic
compounds (oil fractions, phenols, sulphides, PAH etc.) [1,2]. Oil shale semicoke leachate is
characterized by high pH (pH=10-12), high concentration of phenolic (up to 500 mg L-1) and
sulphuric compounds (mainly in the form of sulphate, up to 1700 mg L-1) and its composition
varies according to the weather conditions [3].
Presently the phenol-rich leachate (from 300 up to 10000 m3 day-1) from the drainage system of
semicoke mounds is collected in the ditches at two levels of the dumps. The most polluted part
is treated biologically together with other phenolic effluents and municipal wastewater at the
Kohtla-Järve wastewater treatment plant. The residuary part of leachate is discharged via
channels and the Kohtla and Purtse rivers into the Baltic Sea without treatment. The pollution
from depository deteriorates not only the streams (the Gulf of Finland with phenols, rivers with
PAHs), but also the underlying aquifers [4,5,6].
Due to the difficulty of treating such a leachate, new technologies and new combinations of
various methods are being investigated. Chemical oxidation is known to be a powerful method
for the remediation of wastewater, where a reactive chemical species such as the hydroxyl
radicals (•OH) are generated. A promising alternative to complete oxidation of bio-recalcitrant
wastewater is the use of advanced oxidation processes as pre-treatment to convert initially biorefractory compounds to more readily biodegradable intermediates, followed by biological
oxidation of these by-products [7].
Ozone is known to be capable of destroying recalcitrant compounds in effluent and resulting in
generally less complex products, constituted of smaller molecules and more easily
biodegradable than their precursors [8]. Hence, pre-ozonation can reduce the retention time
required for biodegradation, which represents a feasible improvement in the efficacy of the
leachate treatment process.
The Fenton reagent applied for wastewater treatment processes are known to be a very
efficient tool to remove many hazardous organic compounds from water and overcome the
limitations of bioremediation in degrading toxic and biorefractory organic compounds by
increasing a content of hydrophilic substances, saturating water with oxygen via hydrogen
peroxide decomposition, and breaking down complex compounds into simpler ones that are
easier to degrade [9].
Accordingly, the present study was conducted to investigate the ability of ozonation and
Fenton/Fenton-like treatment to reduce oil shale semicoke leachate COD and acute toxicity.
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International Conference Ozone and UV, April 3rd 2006
During experiments the potential of chemical oxidation to be used as a pre-treatment step to the
following biodegradation was also examined. Thus, BOD/COD ratio was used to measure the
biodegradability of leachate samples.
Material and methods
The leachate samples (A and B) studied were from the oil shale semicoke dumping area of the
oil shale processing company Kiviter, Kohtla-Järve, Estonia. The leachate was collected from
the ditch surrounding the dump site within the time period from November 2004 to January
2005. Samples were stored in darkness at 4°C. No significant changes in the chemical oxygen
demand of the leachate samples could be seen during storage. General properties of the
leachate are shown in Tabel 1.
Table 1. General characteristics of the initial leachate.
Parameter/Sample
leachate A
leachate B
COD, mg L-1
775
1220
BOD, mg L-1
320
630
2-1
SO4 , mg L
1200
830
Cl-, mg L-1
995
810
pH
9.4
10.4
EC50 for Daphnia magna
27.5
15.5
(% of sample)
The Fenton treatment of oil shale semicoke leachate was carried out under batch conditions.
0.2 L samples of leachate were treated during 24 hours in a cylindrical glass reactor under
vigorous magnetic-stirring. Firstly, Fe2+ solution (prepared by dissolving FeSO4·7H2O in twicedistilled water) was added and afterward the reactions were initiated by adding H2O2 all at once.
The molar ratio of H2O2:Fe2+ was kept invariable 10:1, which is the optimal ratio between
hydrogen peroxide and ferrous ions [10]. For the Fenton experiments pH of leachate sample
was adjusted to pH=3. Fenton-like treatment was carried out without pH regulation. The
adjustment of pH was done with 0.1 and 1.0 N solution of H2SO4. After Fenton/Fenton-like
treatment the pH was adjusted to roughly 9.0 by adding 40% aqueous solution of NaOH.
Precipitated iron ions were separated from leachate by means of filtration through a paper (blue
ribbon) filter.
The ozonation experiments were carried out in a semi-continuous bubble column 280 mm in
height and 100 mm in diameter. A porous glass ozone diffuser (35 mm in diameter, porous size
0.1 mm) was located in the centre of the column’s bottom. Ozone, produced from oxygen by a
laboratory ozone generator, was bubbled through the leachate during 180 min. The volume of
leachate sample in reactor was 0.8 L. No pH adjustment was applied in ozonation trials. In
conducted experiments concentration of ozone in feed-gas was kept at 12.0 ± 0.5 mg L-1 and
the gas flow rate at 1.0 L min-1. The inlet and outlet concentrations of ozone in the gas phase
were measured using a Heλion-β (Thermo Electron Corporation, UK) spectrophotometer (λ=258
nm).
All experiments were duplicated and the data on the untreated (initial) samples of the leachate
was verified by the analysis of at least four replicates. The experiments were carried out at 20 ±
1 ºC.
The concentration of sulphate and chloride ions was measured with ion chromatography
according to [11]. The absorbency of leachate samples at 254 nm representing the aromatic
carbon content was measured using Heλion-β spectrophotometer. The concentration of hydrogen
peroxide was measured spectrophotometrically (Heλion-β) at 410 nm as a complex with Ti4+ [12].
The chemical oxygen demand (COD) was determined by closed reflux titrimetric method [13].
Additionally, the 7-day biochemical oxygen demand (BOD7) for the treated and untreated
leachate was determined [13].
Acute toxicity of the initial and treated leachate to Daphnia magna (Cladosera, Crustecea) was
studied and a 24-hours toxicity test was carried out [14]. The medium effective concentration
values (EC50) and their 95% confidence limits were determined for the leachate samples.
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International Conference Ozone and UV, April 3rd 2006
Results and discussion
Ozonation, Fenton and Fenton-like treatment were applied to oil shale semicoke leachate to
reduce COD, acute toxicity and improve biodegradability of the samples. The chemical oxygen
demand is one of the most important parameter of the monitoring of pollution and,
consequently, treatment of wastewater. Thus, COD was examined as the main measure of the
leachate treatment process efficacy. COD of the untreated leachate was 775 and 1220 mg L-1
for samples A and B, respectively. As it is presented in Figs. 1 and 2, the application of
Fenton/Fenton-like treatment and ozonation resulted in a significant reduction of COD (only in
the case of leachate B COD removal during ozonation was quite moderate).
100
COD/CODinitial, %
80
60
40
leachate A
leachate B
20
0
0
100
200
300
400
500
600
700
800
-1
Ozone consumed, mg L
Figure 1. COD residual during ozonation of leachate A and leachate B.
Thus, 63 and 38% reduction of COD was achieved during 180 min ozonation of leachate A and
B, respectively. Moreover, 72-86% of COD was removed by Fenton/Fenton-like treatment of
leachate samples.
100
leachate A - Fenton
leachate A - Fenton-like
COD/CODinitial, %
80
leachate B - Fenton
leachate B - Fenton-like
60
40
20
0
0
0.5
1
1.5
2
2.5
3
H2 O2 /COD
Figure 2. COD residual after Fenton/Fenton-like treatment of leachate A and leachate B.
Also, it was determined that the concentration of sulphate and chloride ions after chemical
treatment increased till a constant value. That indicated the oxidation of sulphur in reduced form
to sulphate and transfer of attached to organic compounds chlorine to free chloride ions. In
addition, the absorbency of leachate samples at 254 nm representing the aromatic carbon
content was measured. It was ascertained that during 180 min of ozonation process the
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International Conference Ozone and UV, April 3rd 2006
absorbency at 254 nm was reduced by 83 and 69% for leachate A and B, respectively.
Fenton/Fenton-like treatment resulted in 81-92% aromatic carbon content removal in oil shale
semicoke leachate. Thus, the content of aromatic organic compounds in leachate was notably
reduced after chemical oxidation.
The biodegradability (BOD/COD ratio) of chemically treated and untreated leachate samples
was also determined (Fig. 3). It was ascertained that both ozonation and the Fenton treatment
led to improvement of the leachate biodegradability. BOD/COD ratio of leachate A was more
effectively increased by ozonation compared to the Fenton process. However, in the case of
leachate B, Fenton-treated sample showed 0.96 BOD/COD ratio value, what means that water
sample is almost completely biodegradable.
1
BOD/COD
0.8
0.6
0.4
0.2
0
Initial
Ozonation
Fenton
0.5:1
Fenton
3:1
Initial
leachate A
Ozonation
Fenton
1:1
Fenton
3:1
leachate B
Figure 3. BOD/COD ratio for untreated, ozonated and Fenton-treated samples of leachate A and leachate
B (ozonation during 180 min, H2O2/COD is weight ratio).
6
EC50 /EC50initial
5
4
3
2
1
0
Initial
Ozonation
Fenton
0.5:1
leachate A
Fenton
3:1
Initial
Ozonation
Fenton
1:1
Fenton
3:1
leachate B
Figure 4. Acute toxicity to Daphnia magna for untreated, ozonated and Fenton-treated samples of
leachate A and leachate B (ozonation during 180 min, H2O2/COD is weight ratio).
EC50 is the parameter indicating the toxicity reduction during the leachate sample treatment by
chemical oxidation (Fig. 4). It was established that ozonation resulted in no improvement of the
toxicity reduction. Moreover, EC50 after ozone treatment changed the leachate even to more
toxic than initial leachate samples. Thus, the ozonation process could not be applied solely as a
detoxification tool for the leachate treatment. In the case of Fenton-treated leachate, both A and
B samples showed the reduction of toxicity. In addition, it was determined that moderate doses
of hydrogen peroxide turned to be more feasible for the toxicity reduction.
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Hence, Fenton/Fenton-like process proved feasible to reduce hazardous impact of the oil shale
semicoke leachate to environment as a single treatment technique. However, medium
concentrations of reagent were found to be enough to achieve good treatment results, and thus
chemical oxidation could be used as a pre-treatment step with the following biodegradation.
Ozonation turned out to be not a detoxification tool, although was capable to improve
biodegradability, and thus also could be applied in conjunction with other methods.
Conclusion
The ability of ozonation and Fenton/Fenton-like treatment to reduce oil shale semicoke leachate
COD and acute toxicity was investigated. The enhancement of leachate biodegradability by
chemical oxidation with moderate doses was also examined.
Ozonation was found to be moderately effective in COD removal and leachate samples
biodegradability improvement, although it was not a detoxification tool. Evidently, oxidation by
ozone could be successfully applied in conjunction with other methods. The application of the
Fenton chemistry resulted in a high COD removal, at least medium BOD improvement and
remarkable acute toxicity reduction. Thus, Fenton/Fenton-like treatment could be used as a
single treatment technique to improve the quality of oil shale semicoke leachate.
It was ascertained that the application of medium doses of oxidants was enough for successful
treatment. Consequently, the efficacy of leachate treatment could be improved by using
chemical oxidation as a pre-treatment step for the following biodegradation. The combination of
chemical and biological technologies would also change treatment process to more costeffective one.
Acknowledgements
The financial support of the Estonian Science Foundation (grant 6524) is gratefully
acknowledged. We would like to thank Ms. Yekaterina Panova for experimental assistance.
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International Conference Ozone and UV, April 3rd 2006
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